1. Field of the Invention
The present invention relates to an improved composite stem construction and method, and particularly, to a composite stem construction for use in a load-bearing joint-replacement prosthetic device.
2. Description of the Prior Art
Stems for orthopedic implants which can withstand both bending and torsion loads are useful in a variety of orthopedic uses. One type of stem which has received considerable attention in the orthopedic field is for a hip joint replacement device. In basic design, this device includes an elongate curved stem which is adapted for receipt in a cavity formed in the proximal region of a femur, and a spherical head carried on a neck at the upper end of the stem. When implanted in operative position, the device functions as a load transfer member between the pelvis and femur, and as such, must accommodate considerable bending, axial compression and torsional forces applied across the joint to the femur.
Four basic constructions have been proposed previously for hip joint devices of this type. In three of these constructions, the curved stem is adapted for insertion into a bone cavity, and the neck is adapted to support the spherical head, usually via a conical trunion joint. Usually the stem and neck are formed as a single piece, and the spherical head is separately attached to the neck, preferably after inserting the stem into the bone. In one construction the stem and neck are formed as a unitary metal piece from stainless steel or, more preferably, from a cobalt chrome or titanium alloy. An advantage of an all metal construction is that the relatively thick metal stem and neck provide adequate bending and shear strength, so that problems of stem fracture or fatigue are minimal. A disadvantage of the construction is a high degree of stress on certain regions of the bone, and stress protection or shielding in other bone regions. Both high stress and stress shielding can cause bone deterioration and resorption, leading to areas of bone weakness and loss of bone support for the prosthesis.
The related problems of bone stress and stress protection which can occur in a hip joint replacement can be understood from the mechanics of weight load transfer across the hip joint device. Normally, much of the weight load is transferred to the femur near the upper joint region and this load is distributed to and carried by the underlying cortical bone region and the prosthesis stem. The distribution of forces in the underlying cortical region and prosthesis stem region is determined by the relative stiffness--or elastic modulus--of the bone and stem respectively. In normal bone, the elastic modulus of the outer cortical bone region is about 2.5.times.10.sup.6 psi, and that of the softer interior cancellous region is less than 1.times.10.sup.6 psi, so that weight loading forces are carried primarily by the outer cortical region. By contrast, the metal stem region of a prosthetic device, which replaces the soft cancellous region of bone, has an elastic modulus typically between about 15-35.times.10.sup.6 psi, so that much more weight loading is carried by the stem, and much less by the outer cortical bone. In addition to the stress shielding this produces in the bone region adjacent the stem, the high-modulus stem also produces unnaturally high bone stress at the lower or distal tip of the stem, where forces carried in the stem are transmitted to the bone.
In a second known prosthesis construction, the stem and neck are formed from rolled or laminated layers of a composite material containing oriented carbon fibers embedded in a polymer resin. This construction is described generally in U.S. Pat. No. 4,892,552, which issued on Jan. 9, 1990, entitled "Orthopedic Device". In a preferred embodiment described therein, a series of composite layers containing fibers oriented in different directions are laminated, according to known composite block construction methods, to produce a machinable block whose different fiber orientations confer strength in different, selected directions with respect to the long axis of the block. The laminated block is then machined to produce a stem and neck piece which can be implanted in bone and fitted with a ball-like joint member. Since the laminate structure has a somewhat lower average elastic modulus, both in tension and shear, than a comparable size metal prosthesis, the above problems related to stress protection along the length of the prosthesis stem, and the high concentration of forces at the distal tip of the stem are somewhat reduced. However, the effective elastic moduli of the stem in tension and shear is still very high compared with the soft cancellous region of bone which the stem has replaced. Furthermore, the laminate material is generally not as strong as a comparable size metal stem, particularly at the neck region of the device where weight loading is borne entirely by the prosthesis. This is due in part to the fact that the carbon fibers oriented lengthwise in the stem do not follow the curvature of the stem, and generally do not extend along the entire length of the stem.
A third prosthesis construction which has been proposed in the prior art involves a metal core having a relatively large-diameter stem which is encased in a low-modulus polymer. A prosthesis of this type is described by Mathis, R., Jr., et al in "Biomechanics: Current Interdisciplinary Research" (Perren, M., et al, eds.) Martinus Nijhoff, Boston (1985) pp. 371-376. The combined modulus of the polymer and inner core of the device is much more like that of interior cancellous bone than is either a solid metal or laminate composite structure, and as a result, problems related to bone stress protection and high stress are reduced. This compound device has not been entirely satisfactory, however. One problem which has been encountered is fracturing at the neck/stem interface, due to large loading force applied at this juncture by the neck. A second problem is related to the cutting action of the relatively stiff metal core against the low-modulus polymer, in response to forces exerted on the stem in directions normal to the stem's long axis. Over an extended period, the cutting action can lead to core wobbling within the bone, and exaggerated movement of the core in response to loading.
In a fourth prior art device which is described in U.S. Pat. No. 4,750,905, which issued on Jun. 14, 1988, an elongate stem is designed to support a load capable of applying both bending and torsional load forces. The stem generally includes an elongate composite core formed of continuous-filament fibers oriented substantially along the length of the core and embedded in a polymer matrix. Where the core has a curved stem, such as in a hip joint replacement device, the fibers extend in a substantially uniform-density, non-distorted configuration from one end of the core to the other. The core is characterized by high tensile strength and elastic modulus, but relatively low shear strength and modulus.
The core is encased in a sheath which encases the stem and tapered section of the core, but not its upper neck. The sheath is made of braided or woven filaments which encircle the stem in a helical pattern extending along the encased portion of the core. The sheath filaments are bonded to the core by a thermoplastic polymer which is infused into the sheath and heat fused to the core. The polymer which embeds and bonds the sheath to the core is part of a thick polymer skin which forms the shape of the implant which fills the space of a bone cavity in which the device is received.
The problem with this device is that the bending modulus along the stem is fairly constant which can lead to higher than desirable stresses in some localized areas. There has been a need to find a simple way to make the stem stiffer in some areas and more flexible in others.
The implant of the present invention solves this problem by providing a stem with a different elastic modulus at different points along the length of the stem. This is done by placing a reinforcing outer wrap at the surface of the implant and varying the orientation of the reinforcing fibers of the outer wrap along the stem length.
In a circular structural member, it is the outer fibers which are most effective in providing resistance to bending and torsion, and which carry the major portion of the stress in doing so. The role of the outer wrap is to provide the hip prosthesis with the major resistance to bending and torsion required to achieve a design having the desired transfer index and design factor as defined hereinafter. The required contribution of the outer wrap to the desired rigidity and strength in each region of the prosthesis is accomplished by varying the orientation of the fibers in the wrap or the thickness of the wrap or both in that region. The outer wrap continues proximally out into the neck region so that joint loads applied to the neck can be transferred rapidly and smoothly to the outer wrap of the prosthesis body without having to be transmitted through the core of the stem below the neck. This is especially important when the outer wrap contacts cortical bone.
The core region of the stem of the present invention consists of unidirectional fibers in a matrix, aligned along the longitudinal axis of the core. The primary function of the core is to provide a strong, stiff neck. The core extends well within the body of the prosthesis in order to securely anchor the neck. The core is used also, although to a lesser degree than the outer wrap, to adjust the rigidity and strength of the body of the prosthesis to achieve the desired stem flexibility.
A filler region is located between the core and the outer wrap and is composed of a material having reduced structural rigidity. This region can act as a mandrel for fabricating the outer wrap. Because the filler contributes little to the overall rigidity of the prosthesis, it permits greater flexibility in adjusting the thickness (number of layers) of the outer wrap to achieve a desired rigidity and strength while maintaining a desired shape. The filler also helps define the shape of the prosthesis for proper fit and transfers stress from the core region to the outer wrap region.